Since an early date, as evidenced by the patent to Thienhaus, U.S. Pat. No. 2,115,129, there have been proposals for the use of saturated vapor-liquid systems in loudspeaker enclosures, using a low boiling temperature liquid. Thienhaus pointed out that condensation and evaporation effects occur, during movement of the diaphragm of the speaker relative to the enclosed volume in which the gas is contained, but his conclusion that restoring forces acting upon the diaphragm are beneficially modified as a result of condensative effects is not universally correct, as will be shown. If was of course known to employ glass fibers in a loudspeaker enclosure, as shown in the patent to Boudouris, No. 2,718,931, which utilizes an acoustically transparent film about the loudspeaker enclosure for other reasons, and the patent to Villchur, No. 2,775,309. The patent to Sullivan, No. 2,797,766, is of interest but utilizes a fundamentally different approach from Thienhaus, in that Sullivan proposes the use of a very heavy gas within a sealed loudspeaker enclosure, to improve low frequency response by reducing the sound velocity within the enclosure, thereby affecting the Helmholtz resonance frequency. Dutch patent No. 111,477 to Kleis of July 15, 1965 proposes the use of a liquid vessel within a separate, interior enclosure in a loudspeaker enclosure, with servo control of the temperature of the low boiling point liquid being used to prevent temperature variations. A fibrous (glass fiber) cylinder is disposed separately from the liquid supply but in the volume of the vapors. The temperature control and the use of a glass fiber structure are for the purposes of minimizing temperature variations and preventing the liquid-vapor system from dropping below a certain temperature.
A similar approach is shown in the patent to Ott, No. 4,004,094, in which a liquid is disposed within a loudspeaker enclosure, in a gas sealing bag, the liquid being held at a specific temperature by a sensing and control servo system, whose sensing means is different from that of Kleis. Ott suggests that the surface of the structure within the vapor space may be increased by the addition of steel wool, and a related suggestion can be found in Dutch patent No. 111,477. Ott specifies that the material of the container should have a high specific heat, for reasons which are not fully specified but which appear in any event to be in error. A related structure is taught by Czerwinski in patent No. 4,101,736, except that the gas-liquid system is supported in a cocoon or bag and a sound absorbent material of fibrous character (fiber glass) is loosely contained within the bag for "heating the fiber by excitation". This statement is apparently to be taken to mean that sound pressure waves absorbed in the fiber glass are supposed to be converted to heat, so as to supply heat to the system.
The teachings of these patents are all based upon the assumption that a liquid sump of a low boiling point liquid will fill the enclosure with vapor and that an efficient interchange between sound pressure waves, the vapor, and the sump liquid will result. The Dutch patent, the patent to Ott and the patent to Czerwinski all suggest that the presence of fibrous materials within the volume above the liquid will be beneficial, but for different reasons, none of which are explained in detail. It has been discovered, however, than when a thermodynamic energy interchange is involved between a gaseous and a liquid state of the same constituent, evaporation and condensation from a liquid sump is not efficient. Further, it is desirable to achieve, in practical applications, the closest approximation to theoretical efficiency that the system will provide, and it is evident that the prior art has not directed its attention to consideration of these aspects. It is apparent moreover, that a fibrous structure such as steel wool or fiber glass also acts to block transmission of sound waves, simply by functioning as an effective sound barrier. Thus filling an enclosure with fibers as suggested in prior patents is also contrary to some fundamental purposes of the vapor-liquid system. For these and other reasons discussed hereafter the beneficial effects of prior art systems have been sharply limited. The only known commercial application of the prior art is a line of loudspeaker systems due to Cerwinski of CERWIN-VEGA known as "Thermo-Vapor" (T.M.), whose interior volume compliance is about the same as dry, glass fiber systems due to Villchur of Audio Research Co. Neither class of systems achieves system compliance as good as would occur for an isothermal system of dry gases.